Certain optical devices, such as a computer mouse, a track ball, a digitizer or a graphics tablet may be used to present a data input signal into a computer. In the example of a computer mouse, a central ball is generally coated with, or made from, a plastic or gum material which rotates as the mouse is moved. The movement of the ball is then transmitted into two small rollers that are generally perpendicular to each other. Each of the rollers converts a direction of movement of the device into an X and Y directional component by transferring the motion to two small disks having small holes. Each of the disks alternately closes or opens a photo sensor assembly that, when rotated, corresponds to the general movement of the mouse in the X or Y directions. The number of interruptions and releases of the photo sensor assembly is generally a function of a quantity equal to the amount of movement of the mouse in the particular X or Y direction. The number of interruptions and releases per second defines a functional quantity equal to the speed of the movement of the device. Other input devices function in a similar fashion by converting directional information into X and Y components using optical components.
Conventional approaches implement circuits for producing fixed bias currents for each direction (i.e., the X and Y directions) of movement of a pointing device. The fixed bias currents in conventional devices can be provided by a fixed resistor. A circuit with a fixed or variable threshold may be used to sense the state of a photo transistor. Another conventional approach uses a pulse width modulated bias current on an LED to vary the LED light output and hence the photo transistor response.
FIG. 1 illustrates a circuit 10 comprising a conventional fixed bias current device. The circuit 10 has a photo transistor (PTR) 12 and a sensing section 14. The sensing section 14 has a buffer 16 and a resistor 18. A primary disadvantage with the circuit 10 is that it does not generally tolerate a wide variation in the properties of the optical components such as the photo transistor 12. Additionally, the circuit 10 does not allow for dynamic optimization of the system performance after fabrication. For example, in the two extremes, an out of tolerance photo transistor 12 may be in a saturation mode due to the light being too bright, or may be in a cutoff mode if not enough light is present to be greater than the threshold necessary to turn on the photo transistor 12. Since each of the components of the circuit 10 is fixed, adjustments for variations in system components are not practical.